Not Applicable
Not Applicable
The invention relates to the field of fiber optic communications systems, and in particular to wavelength division multiplexed transmission systems and networks.
There is a pressing need for increased transmission capacity over installed optical fibers. One effective commercialized method to achieve greater transmission capacity is to use wavelength division multiplexing (WDM), a technique for simultaneously transmitting several optical signals on a single optical fiber. There are a large number of installed WDM transmission systems throughout the world. All of these installed systems are limited in the number of wavelengths or channels they can support and the data rate at which individual channels can be modulated. These limitations arise from system losses, fiber non-linearities, chromatic and polarization dispersion, polarization dependent losses, coherent and incoherent crosstalk, and performance limitations of installed optical amplifiers.
In WDM transmission systems, the information-carrying optical signals are transmitted on end-to-end links. Each link is typically divided into numerous segments or spans, and signal regeneration technology is used between spans to provide amplification and other signal compensation to overcome losses and distortion from the preceding span. There are a variety of types of regeneration technologies. They can be purely electronic, purely optical, or a combination of both. One type uses an optical amplifier, and is referred to herein as an amplifier node. A known type of optical amplifier in widespread use today is the Erbium Doped Fiber Amplifier, or EDFA. Other types include Raman amplifiers (bulk and distributed) and semiconductor optical amplifiers. Amplifier nodes can also include compensation components, such as dispersion compensating fibers to compensate for distortion such as chromatic dispersion, and polarization mode distortion (PMD) compensators. Amplifier nodes can also contain gain equalization components to adjust the gains of the individual wavelengths of the system, as well as optical spectrum monitors to monitor the power, wavelength, and quality of the individual optical wavelengths.
Many of the performance characteristics of existing optical communication systems are influenced by the characteristics of basic system components, such as the fibers and the optical amplifiers, which are expensive hardware components intended for long and continuous service. Important fundamental characteristics include the gain and saturated output power of the optical amplifiers, and the losses, dispersion, non-linearities, polarization-dependent losses, channel isolation, and PMD caused by the fiber spans and optical components within the system. Existing optical communication equipment has been designed in a manner that reflects the characteristics of these components. Key system parameters include the maximum number of channels or wavelengths per fiber, the frequency or wavelength spacing between channels, and the data rate and optical power level of each channel.
Thus, upgrading a system to take advantage of newer technology can involve significant expense, as well as disruptions in service, to change amplifiers, fibers and/or other basic system components. For example, a known type of existing system is a WDM OC-48 system, where OC-48 refers to a standard optical signal format for transmitting data at 2.488 Gb/s. Upgrading some or all channels of a 32-channel WDM OC-48 system to OC-192 (a 9.952 Gb/s signal) can require replacing each optical amplifier in the system with a new amplifier having 6 dB more output power, in order to retain desired signal quality. Optical amplifiers can cost on the order of $100,000 apiece, so the cost of such an upgrade can be substantial. Furthermore, there is no guarantee that the fibers can carry the required additional power without causing unacceptable signal degradation due to fiber nonlinearities. Non-linearities in other system components (e.g. dispersion compensating fiber) may also degrade the signal unacceptably. Non-linearities tend to increase with optical signal power, so that in some cases 6 dB more signal power may cause in excess of 12 dB more fiber non-linearities. Adequate optical regeneration of the degraded signal may not be possible. Thus, there are significant obstacles to be overcome to increase transmission capacity in optical communication systems.
It has been known to use forward error correction coding (FEC) of optical data signals to improve WDM transmission systems. One technique is shown in a paper written by Livas et al. entitled xe2x80x9cForward Error Correction in a 1 Gb/s/Channel Wavelength-Division Multiplexed Systemxe2x80x9d, Proceedings of the IEEE Lasers and Electro-Optics Society Summer Topical Meeting on Optical Networks and Enabling Technologies, Lake Tahoe, Nev., July 11-13, paper W2.5, 61-62 (1994). This paper shows that the use of FEC enables the system to better tolerate channel-to-channel crosstalk, so that channel spacing can be reduced without compromising performance.
Different results are shown in a paper by Puc et al. entitled xe2x80x9cConcatenated FEC Experiments Over 5000 km Long Straight Line WDM Testbedxe2x80x9d, Proceedings of the Optical Fiber Communication Conference, OFC 1999, San Diego, Calif., February 20-25, paper ThQ6, p 255, (1999). This paper shows that the use of FEC can improve performance in undersea systems having no repeaters. In particular, Puc et al. show that a system designed with coded signals can have better system noise margin and reduced pulse distortion in long optical amplified digital transmission systems. In addition, this improved margin can be used to design the system with increased amplifier spacing and/or increased system capacity and/or decreased channel spacing.
U.S. Pat. No. 5,715,076 to Alexander shows a system in which selected channels have remodulators 30 and remodulation selectors 100 that include FEC coders/decoders. The use and benefit of FEC coding are described in column 6 from line 8 to line 38. While Alexander shows the use of FEC, it does not teach how to utilize FEC to increase the transmission capacity in an installed WDM transmission link.
This previous work has not focused on the problems associated with upgrading an existing and installed WDM transmission system.
In accordance with the present invention, a method and apparatus are disclosed for upgrading existing optical communications systems to provide increased transmission capacity without incurring the expense of replacing optical amplifiers or fiber spans. Disclosed data transmission apparatus includes first and second optical transmitters coupled to an optical link. A forward error correction (FEC) coder is coupled to the input of the second optical transmitter. A first information signal having a first information rate is provided to the first optical transmitter, and a second information signal having a second information rate is provided to the second optical transmitter. The use of the FEC coder enables the second information signal to have a higher information rate than the first information signal, while simultaneously enabling the respective output power levels of the first and second optical amplifiers to be substantially equal. The data transmission apparatus achieves higher data transmission capacity while retaining compatibility with existing fiber spans and amplifier nodes.
A disclosed optical transmission system incorporates an optical transmitter and an optical receiver having certain transmission characteristics, such as output power level, decision threshold level, etc., that are adjustable in response to bit error rate information. A bit error rate estimator is provided at the receiving end of the optical link. The bit error rate estimator estimates the bit error rate of the received information signal, and provides bit error rate information to the optical transmitter and/or the optical receiver and/or elements within the transmission link based on the estimated bit error rate, in order to optimize the performance of the transmission system.
Other aspects, features, and advantages of the present invention are disclosed in the detailed description that follows.